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A tiny furry animal that lived in the age of the dinosaurs may be the closest fossil relative of living mammals.

American and Chinese researchers have uncovered a 12 millimetre-long fossilised skull of the creature which lived 195 million years ago and is thought to have weighed no more than a large paper clip.

Mammals are the group traditionally defined as warm-blooded animals whose young are nourished by milk from the mammary glands of the female parents.

But the new fossil, called Hadrocodium, has other crucial mammalian features including three middle ear bones and a single lower jaw bone. These bones appear in the tiny skull 40 million years earlier than seen in other ancient fossils.

The researchers' comparison of 90 features of the tiny skull with other early mammal fossils and with living mammals suggests the new species is the closest known fossil relative of the living mammals.

This is valuable to biologists because it sheds lights on the stages of evolution of the mammal skeleton.

The well-preserved skull, described in the latest issue of the journal Science, shows the unique skeletal features of mammals probably evolved step by step.

The scientists, led by Zhe-Xi Luo of the Carnegie Museum of Natural History, estimated the animal's mass (around two grams) by comparing its skull size with other mammals. It appears to be one of the smallest mammals ever known. This and its distinctive teeth suggest it probably fed on insects.

The fossil was discovered in 1985 in China but its significance was realised only recently. After meticulous work removing the sediments encasing it, it was clear its anatomy was completely different from any other mammal known from the time period.